The present invention relates to a digital signal processing system, and more particularly to, a digital signal processing system for error correcting with respect to the moving image between continuous image frames, and replacing the image in the uncorrected region with the compensated preceding image signal, to thereby perform error concealment.
In general, the image data compression technique is considered the heart of image digital signal processing, whose application extends throughout the fields of picture conference systems, video telephones, multimedia, digital video cassette recorders and digital hi-definition television. In combination with the image data compression technique, many algorithms have been developed for transmitting or recording the best quality image within restricted channels.
For compressing the image data, a method is used whereby correlation which exists in the images is utilized to effectively reduce the amount of data. Particularly, "two-dimensional image compression" refers to the compression using the correlation within a frame, and "three-dimensional image compression" refers to compression using the correlation within one frame and between successive frames.
To use the two-dimensional correlation of images, a transmitting coding is generally used. Among many transmitting coding methods, a discrete cosine transform (DCT) method is the one most often used in recent years.
Meanwhile, the correlation between frames in the three-dimensional image compression is affected by the degree of motion belonging to the image. This motion is detected, and the image difference between frames which is extracted according to the detected motion, is encoded.
The digital image apparatus adopting these data compression techniques uses error correction and error concealment methods to prevent picture quality degradation due to errors generated in recording (or transmitting) and reproducing (or receiving).
Here, the error correction is to correct the errors included in the reproduced data (received data). For this end, error correction codes are utilized, and more particularly, the product code has been in a wide use for correcting the burst error.
Error concealment is to replace the image region which is not corrected even after performing error correction, with the image signal of the preceding field which does not have the error to thereby reduce the picture quality degradation.
FIG. 1 shows the conventional digital signal processing system established with the error correction and error concealment functions.
Error correction decoder 10 corrects the errors in the data transmitted from the recording or transmitting portion. Error correction decoder 10 includes an inner error correction decoder for correcting the errors which are within the correction capability and attaching an error flag to the errors which exceed the correction capability; a de-interleave processor for performing the de-interleave treatment on the interleave-processed data at the transmitting or recording side; and an outer error correction decoder for correcting again the data attached with the error flag among the output from the de-interleave processor, and, if the error is not yet corrected, attaching again the error flag to the error signal to be output.
Error correction techniques are disclosed in the Japanese Laid-open Patent Publication No. sho 64-30344 and Korean Patent Application No. 91-15248.
A data expander 20 expands the data being output from error correction decoder 10 to thereby restore the original signal. In addition to the compression structure (although not shown) of the transmitting or recording side, which includes a quantizer for performing DCT processing, a variable-length encoder and an error-correction encoder, the data expander further comprises a local decoder performing art inverse quantization and inverse DCT function, a frame memory, a motion predictor and a motion compensator for the utilization of the three-dimensional image correlation generated between frames, to thereby have a three-dimensional image compression structure.
As for the three-dimensional image compression structure, while the intra-frame processing is performed in order to code using the correlation within one frame, the local decoder stores intra-frame data to detect the motion of the succeeding frame.
Next, the frame data which is processed based on the inter-frame information and the preceding frame's data read out from the frame memory are compared with each other so that the motion between the two frames is detected. Also, the prediction data according to the detected motion is extracted from the motion compensator, so that the prediction error, which is the difference between the current frame data and the motion-compensated data, is DCT processed and encoded.
Accordingly, in the case of intra-frame decoding, data expander 20 performs the decoding by way of an error correction decoder, a variable-length decoder, an inverse quantizer, and inverse discrete cosine transform means. At this time, the intra-frame data is stored in the frame memory.
The ensuing inter-frame processing is for performing the motion compensation according to the motion vector decoded in the variable-length decoder, to thereby restore the prediction error after the inverse-discrete-cosine-transform processing.
Post processor 30 de-scrambles the scrambled data and changes the ratio of the luminance signal and color difference signals from 4:2:0 to 4:2:2. This is because, at the transmitting or recording side, the video signal having the ratio of 4:2:2 (for example, Y: 720 pels.times.480 lines, R-Y: 360 pels.times.480 lines, and B-Y: 360 pels.times.480 lines) is converted into video signal data having the ratio of 4:2:0 (for example, Y: 720 pels.times.480 lines. R-Y: 360 pels.times.240 lines, and B-Y: 360 pels.times.240 lines) via a pre-filter in the line sequence of vertical subsampling, and because the scrambling process is performed before the discrete cosine transform, to reconstruct the image so that the relatively consistent parts (those portions bearing little change in the image data) and changing parts (those portions bearing greater change in the image data) are spread uniformly.
Error concealer 40 restores the parts having the errors exceeding the error correction capability, by restoring the compressed picture image and using the picture information of the preceding frame.
A digital-to-analog (D/A) converter 50 converts the picture signal being output frown error concealer 40 into analog form, and thereby displays an analog picture signal on the display unit such as a monitor.
Here, the error concealer can further comprise a noise remover detecting the motion coefficient, i.e., the difference of image information between the current frame and the preceding frame, in the case of a motion picture, for outputting the output of error concealer 40, in the case of a still picture, for outputting the picture signal of the preceding frame, and, in the case of a picture having a certain degree of motions, for outputting as the picture signal the mixed signal of the current frame's picture signal and the preceding frame's picture signal.
FIG. 2 is a block diagram of the detailed error concealer shown in FIG. 1.
Referring to FIG. 2, the error concealer comprises a first frame memory 41 for storing the luminance signal of post processor 30 by frames: a second frame memory 42 for storing the chrominance signal of post processor 30 by frames; a first address generator 43 for generating the addresses of first and second frame memories 41 and 42; a first-in-first-out (FIFO) memory 46 for storing the error flag output from error correction decoder 10 by symbol in order to set the timing in accordance with the image data from post processor 30: a second address generator 47 for generating a de-scrambled error flag address; a third frame memory 48 for storing the error flag which is stored in FIFO memory 46 by frames according to the address generated by second address generator 47; and first and second multiplexers 44 and 45 for selecting the output of post processor 30 in accordance with the error-corrected region and selecting the image signal of the preceding frame stored in first and second frame memories 41 and 42 in accordance with the picture region where the error-correction has not been performed, according to the error flag signal being output from third frame memory 48, to thereby transmit the respective outputs to D/A converter 50.
As for the digital signal processing system shown in FIGS. 1 and 2, the error treatment is inevitable in the digital processing of the image signal. For performing such error treatment, error correction coding is utilized. However, the error portions, such as burst error, which cannot be corrected by the error correction decoder, will cause a significant degradation in the picture quality. Here, an error concealment is adopted to reduce the picture quality degradation. The conventional error concealment method is to merely perform replacement by borrowing data which corresponds to the error location in the current france (or field)from the stored, preceding frame (or preceding field) only.
Such a conventional method causes a picture quality degradation and makes an image bearing a greater degree of motion appear unnatural. Therefore, a more effective method for improving tile present problem becomes necessary.
For example, in a digital video cassette recorder having many reproducing methods such as normal playback, high-speed playback, and slow and still reproductions, when a picture signal is recorded in digital form, tile still and slow reproduction modes reproduce repetitive picture data (or stores the reproduced data in the memory and reads out the data to be processed), so that a picture is displayed on a screen.
Here, the reproduced data should be error-correction encoded. However, as for the portions in which the errors are not corrected, just the preceding frame's picture signal is restored at the error concealment process. Due to this, the picture signal of the preceding frame having the portions where the errors are not corrected, causes the succeeding pictures to have the same errors at the same positions, so that the picture quality is degraded. Accordingly, the original picture quality cannot be obtained.